Combined cycle power systems include one or more gas turbines and heat recovery steam generators (“HRSGs”) and a steam turbine. Traditional combined cycle system startup procedures include low load limits of the gas turbine and restrictions on the gas turbine loading rate to control the rate of increase in steam temperature. These limits and restrictions contribute to air emissions during the startup event, may increase starting and loading time, and may increase fuel consumption during starting and loading.
Recent improvements to combined cycle power systems allow the gas turbine to be started and loaded independently from the start of the steam turbine by utilizing both parallel and cascading steam bypass paths for steam pressure control and terminal attemperators for steam temperature control. As a result of these recent improvements, the gas turbine can be loaded to base load as quickly as possible independent of the steam turbine starting requirements. Therefore, it is possible that steam generation can reach much higher levels early in the startup.
To increase power generation in early startup it becomes beneficial to consume as much of the generated steam as possible. However, steam turbine constraints (e.g., stress, differential expansion, clearances, etc.) may limit the rate at which the steam consumption can rise, without comprising any of these steam turbine constraints, namely excessive moisture. Similar principles apply when loading other steam systems and controlling steam temperatures apart from combined cycle power systems, such as steam turbines and boilers.
Thus, there exists a need for improved methods and systems for loading a steam turbine.